1. Field of the Invention
The present invention relates to semiconductor devices and wire bonding methods therefor. The present invention also relates to manufacturing methods of semiconductor devices using lead frames.
This application claims priority on Japanese Patent Application No. 2006-221457, Japanese Patent Application No. 2006-240005, and Japanese Patent Application No. 2006-280313, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. H06-302638 teaches an example of a semiconductor device in which bonding wires are arranged so as not to come in contact with each other, wherein electrode pads of a semiconductor chip are each electrically connected to leads arranged in the periphery of the semiconductor chip via wires. In this semiconductor device, when electric currents flowing between the electrode pads and leads increase, the wires are heated and melted, or voids unexpectedly occur due to alloy diffusion at joining portions between the wires and leads.
A conventionally-known method for solving the aforementioned drawback is realized by the constitution in which a plurality of wires are arranged between a single lead and a single electrode pad and each have a reduced magnitude of electric current flowing therethrough. In order to downsize the semiconductor device adopting the aforementioned constitution, wire bonding positions are aligned in a longitudinal direction of the leads. Since a relatively large number of wire bonding positions should be arranged within the limited area of a semiconductor chip, wire bonding positions must be set inwardly of the periphery of the semiconductor chip as well.
However, the aforementioned constitution has a drawback in that electric short-circuiting may occur between the wires, which are arranged very close to each other within the wires connected to the adjacently arranged leads.
Japanese Unexamined Patent Application Publication No. 2004-56021 and Japanese Unexamined Patent Application Publication No. 2005-12140 teach wire bonding methods in which bonding pads formed on semiconductor chips are electrically connected to contacts such as leads of lead frames and electrode pads formed on boards for mounting semiconductor chips via metal wires, wherein a first bonding is performed so that distal ends of wires join one of bonding pads and contacts by way of ball bonding, and then a second bonding is performed so that rear-ends of wires join the other of bonding pads and contacts by way of wedge bonding.
In the first bonding, metal balls whose sizes are larger than the diameters of wires are formed at the distal ends of the wires, which are supplied from the terminal end of a capillary, in advance, so that the terminal end of the capillary presses the metal balls toward one of the bonding pads and the contacts so as to perform compression bonding on the metal balls. In the second bonding, the rear-ends of the wires are held between the terminal end of the capillary and the other of the bonding pads and the contacts and are thus subjected to compression bonding.
Compared with ball bonding, wedge bonding realizes relatively weak joining strengths. For this reason, metal bumps are formed in advance on the other of the bonding pads and the contacts by use of the capillary, and then the rear-ends of the wires join the metal bumps in the second bonding.
Both of the metal balls and metal bumps, which are attached onto the bonding pads and contacts in the conventionally-known wire bonding method, are formed by use of the capillary for supplying wires, wherein they are formed by pressing the distal ends of the wires onto the bonding pads and contacts. For this reason, the sizes of the metal balls and metal bumps become larger than diameters of the wires, wherein the size of a typical example of the metal ball or metal bump is increased to at least two-times larger than the diameter of the wire. In other words, it is necessary to set the sizes of the bonding pads and contacts in conformity with the sizes of the metal balls and metal bumps rather than the diameters of the wires.
When semiconductor chips, boards, and lead frames are used in digital amplifiers, relatively high electric currents flow through wires; hence, it is necessary to use large-diameter wires in establishing electrical connections. This also increases the sizes of the bonding pads and contacts as well.
Due to recent tendencies regarding high integration and downsizing of semiconductor devices, distances between adjacent electrode pads and distances between adjacent leads tend to be reduced, whereby distances between wires are correspondingly reduced. Japanese Unexamined Patent Application Publication No. H06-302638 teaches that electrode pads and leads are each arranged in a zigzag manner, or adjacent wires are varied in loop heights thereof, thus preventing adjacent wires from coming in contact with each other. Herein, the wires join the leads at the substantially same height; hence, electric short-circuiting may likely occur between adjacent wires in proximity to leads.